Summary

In the past many humans regarded the appearance of a comet as an omen or harbinger of doom, but we now know them as time capsules from the formation of the solar system. Created from the gradual accretion of rocks, dust, and ice, comets have been preserved in the “deep freeze” of space far from the Sun, likely retaining in the present much of their formative character. The present-day structure and composition of comets provide a window into the conditions that prevailed during the birth of our solar system. Comet 67P/Churyumov-Gerasimenko, the target of the Rosetta mission, is a member of the Jupiter-family dynamical group of comets that have likely spent most of the last few billion years stored in the ring-shaped disc of icy bodies beyond the orbit of Neptune known as the Kuiper Belt. Subtle gravitational interactions can eventually push icy bodies in the Kuiper Belt reservoir toward the Sun where, under the gravitational influence of Jupiter, they evolve into short-period orbits around the Sun. Once its journey into the inner solar system begins, however, its days are numbered as heating from the Sun erodes the comet in a generally gradual but occasionally dramatic fashion. On pages 1563 and 1566 of this issue, Filacchione et al. (1) and Fornasier et al. (2), respectively, report on the long-term and close-up surveillance of 67P by Rosetta, revealing for the first time both the day-to-day and seasonal evolution of a comet nucleus (1, 2).